1. Field of the Invention
This invention relates to an optical scanning apparatus, a surface-state inspection apparatus and an exposure apparatus, and more particularly, to an optical scanning apparatus, a surface-state inspection apparatus, and an exposure apparatus using such apparatuses which are suitable for detecting a pattern defect, or a foreign particle, such as dust or the like, present on a substrate, such as a reticle, a photomask or the like (hereinafter generically termed a reticle) when they are used in a semiconductor production facility.
2. Description of the Related Art
In the IC (integrated circuit) production process, a circuit pattern for exposure formed on a reticle substrate is in general transferred onto the surface of a wafer coated with a resist using a semiconductor printing apparatus (a stepper or a mask aligner) to produce an IC.
At that time, if a foreign particle, such as dust or the like, is present on the surface of the substrate, the foreign particle is simultaneously transferred in a transfer operation and can cause a defective IC to be produced, thus causing a decrease in the yield of IC production.
Particularly when a circuit pattern is repeatedly printed on the surface of a wafer by a step-and-repeat method using a reticle, one foreign particle on the surface of the reticle is printed on the entire surface of the wafer, causing a great decrease in the yield of IC production.
Accordingly, it is important to detect the presence of a foreign particle on a substrate in the IC production process, and therefore various kinds of inspection apparatuses have been proposed. FIG. 1 shows an example of such an apparatus. Such an apparatus has been described in detail, for example, in Japanese Patent Public Disclosure (Kokai) No. 62-219631 (1987) (U.S. Pat. No. 4,886,975). This apparatus has a feature in that care is taken for shortening the inspection time compared with other conventional apparatuses. That is, an incident light beam 3 passing through an f-.theta. lens 2 is divided into two, i.e., upper and lower light beams by a half-mirror 4. These beams are condensed onto points P and Q on a substrate (reticle) 1 by reflecting mirrors 5 and 10, respectively. The reticle 1 has in general a surface having a circuit pattern at its lower side (1b) and a surface remaining as a glass blank at its upper side (1a). Only the pattern surface may be inspected when inspecting circuit defects, but the light beams must in some cases be incident on both the pattern surface and the blank surface when inspecting foreign particles, such as dust or the like.
A rotating element (polygon mirror, not shown) is provided in front of the f-.theta. lens 2, and scans the incident light beam 3 in a direction orthogonal to the plane of FIG. 1 by the rotation of the polygon mirror, whereby the upper and lower light beams scan the reticle surface in a direction perpendicular to the plane of FIG. 1. In order to inspect the entire reticle surface, the reticle 1 is moved in the direction from S.sub.2 to S.sub.1 in the plane of FIG. 1, whereby raster scanning is performed on the reticle surface. Scattered light issued from the incident point P on the reticle 1 is focused on a field stop 7a by the function of a light-receiving lens 6a. The field stop 7a guides only necessary signal light to an optical fiber 8a and a photomultiplier 9a in the following stage, and cuts out unnecessary flare light other than the signal light.
A light-receiving system 31 for scattered light issued from the incident point Q has the same configuration as described above. That is, a light-receiving lens 6b, a field stop 7b, an optical fiber 8b and a photomultiplier 9b have the same functions as described above.
As the size of a semiconductor chip is being further and further reduced, more accuracy is required for a reticle for producing these smaller semiconductor chips. Accordingly, a more precise inspection function than is available in the conventional art is required for a surface-state inspection apparatus for inspecting the reticle.